National Physical Laboratory

Dose rate

radiation sign

 

The instrument should be calibrated in terms
of Ambient Dose Equivalent. 

The results of calibration may be quoted like this:

IR: Instrument response in terms of ambient dose equivalent, H*(10), using 137Cs (662 keV)

radiation formula
Where  Ri is the observed reading {or rate h-1}

            B is the background reading {or rate h-1}

            H*(10) is the applied 137Cs ambient dose equivalent or ambient dose equivalent rate in µSv or µSv h-1

E.g. If the applied ambient dose equivalent rate is 500 µSv h-1 and the net observed reading (Ri – B) is 550 µSv h-1

radiation formula

 IR= 1.10

This is the response to high energy γ radiation (662 keV) in terms of H*(10)  ambient dose equivalent  rate.

Or

Cf: Calibration factor in terms of ambient dose equivalent, H*(10)

This is the reciprocal of the IR

 radiation formula 

  Cf = 0.91

When measuring high energy gamma radiation (662 keV), the calibration factor is 0.91 (or multiply the net instrument reading by this number to obtain the ambient dose equivalent rate in µSv h-1).

Similar calibration data will be obtained using 241Am (60 keV) and 60Co (average 1.25 MeV). 

Most instruments in common use will have a relatively flat energy response between 60 keV and 1500 keV.  If the instrument will be used to measure energies beyond these, it will be necessary to consult the energy response curve of the instrument provided in the manufacturer’s data and make suitable corrections.

Guidance on the interpretation of manufacturer’s type test data – photon doserate

Often in the workplace, the mean photon energy is likely to be unknown and some assumptions need to be made.

For example, in order to take a measurement of tube leakage of an X-ray set operating at 80 kVp, an assumption is made that the mean energy penetrating the X-ray tube shielding is close to 80 keV. 

A compensated GM tube would be suitable for this application due to its high sensitivity. The manufacturer should provide data on how an instrument performs at different energies; a typical response curve for this type of detector is shown below. The points where a calibration is normally carried out are circled (60 keV and 662 keV) and the response of the detector at other energies is plotted against the relative response to 662 keV.

radiation graph

Figure 1 Typical response curve for a compensated GM detector showing calibration points

Below 80 keV, the rate of change of response is quite dramatic. From the curve above it can be seen that the response at 80 keV is approximately 0.95, or 95 %, of the response at 662 keV.

In practice, many users choose to assume that the indication on the instrument is absolutely correct because the calibration confirmed that the instrument was performing to within ± 30% of its expected response. This may be perfectly acceptable in some circumstances however to reduce the uncertainty of a measurement, the instrument’s relative response to a calibration source may applied.

Using data provided on a calibration certificate for the actual response of the instrument at 662 keV, the correction for the response at 80 keV can be determined.

If the calibration certificate quotes an instrument response, IR of 1.1 for 137Cs, and the instrument’s relative response at 80 keV is 0.95, the estimated response at 80 keV would be:

1.1 x 0.95 = 1.04

The net instrument reading should be divided by 1.04 to give the best estimate of the dose rate at 80 keV.


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